1. Field of the Invention
This invention relates to d-c power supply circuits, and, more particularly, to a circuit for supplying d-c power to a load such as gas discharge lamps.
2. Description of the Prior Art
The operation of tubular fluorescent lamps on direct current is known to improve efficiency in terms of lumens of light output per watt input. This is primarily due to the fact that the low pressure mercury discharge is more efficient with d-c operation in producing resonant radiation with lower current density, when the mercury vapor pressure is in the conventionally used range. However, the efficiency of d-c operation and efficacy in lumens per watt (LPW) when the lamp is operated on d-c current can be affected by the power.factor and ripple factor of unidirectional- current. It has been observed that operating with a power factor in the range of .9 and 1.0 and the ripple factor as low as possible are most desirable for efficient d-c operation.
For d-c operation large inductors were required to minimize the ripple in the d-c output. However, with the larger inductors the losses in system efficiency increase. A discharge lamp ballasting circuit is described in U.S. Pat. No. 3,233,148, issued Feb. 1, 1966 to William H. Lake and assigned to the present assignee. This patent describes several circuit configurations in which inductive impedance elements are added to rectifier-capacitor bridge ballast circuits to achieve a bimodal impedance characteristic for the discharge lamp with a cycle of transitions occurring at twice the power line frequency. The system of the Lake patent provides a power factor typically in the 0.5 to 0.7 range. This power factor is responsible for a significant part of the overall system loss in efficiency. The circuits shown also produced a large ripple factor of 50% or greater.